Add SQL Support to InferColumns

[luisquintanilla]

Currently InferColumns only supports file data sources.

Add SQL support to InferColumns so it can be used with DatabaseLoader.

[luisquintanilla]

Sample code using existing APIs: https://github.com/luisquintanilla/InferColumnsSQL

I think it should be feasible to bring this into the framework to make a cleaner experience.